In order to meet the needs in the recent electronic appliance industry for mounting components in high density in a compact space, the increasing number of printed wiring boards is shifting from the conventional single-sided version towards the double-sided and those having multi-layers. Thus the development efforts are focused on producing a high density printed wiring board that can accept as many wiring circuits thereon.
For forming a fine hole (approximately 200 μm diameter, for example) at a high speed, use of an energy beam, such as a laser beam, is being studied in place of the conventional through-hole making with a drill (Kiyoshi Takagi: “Significant Trends, the Development of Buildup Multilayered PWBs”, Surface Mounting Technology, No.1, pages 2-10 (1997); and other publications). Examples of the board material include a film of thermosetting resin at B-stage state containing uncured component, and a composite material of woven, or nonwoven, organic or inorganic fabric and thermosetting resin.
A conventional process comprises a hole forming step and a connection means forming step, as shown in FIG. 5. The aim of forming a hole in a wiring board is to interconnect the circuits provided on the surfaces or in the inner layers of a wiring board. The interconnection means is formed by filling the hole with a conductive paste or by applying a plating process. For example, a both-surface type printed wiring board is produced by first filling a through-hole of the board with a conductive paste containing electroconductive particles by way of a printing or the like process, and then copper foil is provided on both surfaces of the board to be unitized by thermal compression. The copper foils are then made to have specific patterns. The board material is required to become soft or molten at the thermal compression; so, a B-stage thermosetting resin containing uncured component or a thermoplastic resin is used.
When forming a hole in a wiring board of the above described material with an energy beam, such as a laser beam, a resin 2 and a woven, or nonwoven, fabric 3 contained in the board material 1 are heated by the beam and sublimated to be scattering around, and a hole 5 is formed. Resin material existing around the hole is also softened and molten by a heat of low-energy beam that does not contribute to perforating a hole, which resin material oozing out from the wall surface of the hole forms a thin film to cover a part or the whole of the wall surface, as illustrated in FIG. 6(a).
In a case when the resin material of the board absorbs moisture, it is soften and molten more easily by the heat of an energy beam, and volume of the oozing resin increases. As volume of the absorbed moisture increases, the oozing resin comes into contact to each other as shown in FIG. 6(b), which is then solidified and contracted after cooling to form a thin resin film 10, eventually clogging the through-hole 5. This phenomenon becomes significant with the holes of small diameter.
FIG. 7 is a chart showing a concept of relationship between percentage of water absorbed in the board material 1 and rate of rejects due to formation of resin film 10. As soon as the percentage of water absorbed in the board material (water absorbed versus board material 1 in terms of weight; hereinafter referred to as wt %) exceeds a certain point the reject rate sharply picks up. The reject rate and the percentage of absorbed moisture in the threshold value vary depending on the hole diameter and the board material 1. In the through-holes 5 having such a resin film 10 formed therein, troubles in the electrical interconnection often arise in a subsequent process step. Examples of the trouble include; no electrical connection accomplished between the circuits of both surfaces or layers of a wiring board because of incomplete formation of the conductive material or the plating through the opposite surface of a board, the hole not filled with sufficient amount of conductive material, a high contact resistance in an accomplished interconnection, etc.
The present invention aims to offer a manufacturing method, a manufacturing equipment and a board material for producing reliable printed wiring boards having quality holes. In accordance with the invention, rejects due to the formation of resin film is eliminated, or at least reduced, to an improved production yield rate.